Analysis and prediction of the atmospheric boundary layer characteristics during the NOAA/ARL-JSU meteorological field experiment, summer-2009

Precise prediction of atmospheric planetary boundary layer (PBL) structure at high spatial and temporal resolution is important due its influence on the dispersion of pollutant emissions. The PBL has diurnal temporal variation, and its spatial variations are dependent on topography and land use. In this study, a high resolution mesoscale atmospheric model was used to predict the diurnal variations of the boundary layer over the Gulf Coast region for the period of 16-19 June, 2009 during which special meteorological observations were collected. This study assumes significance due to the complexity of the Gulf Coast region and its influence on the atmospheric dispersion of pollutants and other hazardous material.

Observations collected over the Mississippi Gulf Coast region during June-2009, as a joint NOAA-ARL and JSU-TLGVRC field experiment, were analyzed to understand the characteristics of the planetary boundary layer (PBL). These comprise radiosonde observations collected at 5 times of 1400, 1600, 1800, 2000 and 2200 UTC for four consecutive days from 16 to 19 June, 2009. During this period, radiosonde observations were collected at the two locations, normal to the Mississippi Gulf Coast, of Harrison County School (30.5N, 89.1W) and Wiggins Airport (30.8N,89.13W). The main objective of this field experiment was to study the coastal boundary layer structure associated with the development of sea breeze circulation.

A high resolution mesoscale model, ARW (Advanced Research WRF), was used to simulate the mixed layer characteristics of the PBL over the Gulf Coast Region corresponding to this 4-day period. The model was designed to have nested two-way interactive three domains with 36, 12 and 4 km resolutions, with the inner most domain covering the entire Gulf Coast region, and 41 vertical levels of which 30 levels were chosen to be below 500 hPa level so as to fine resolve the boundary layer features. The initial and boundary conditions were provided from NCEP FNL data available at 1 degree interval and the boundary conditions were updated at every 6 hours. The model was integrated for 48 hours starting from 00 and 12 UTC of each day starting from 15 June up to 00 UTC of 18 June 2009.

The model simulated atmospheric fields were used to derive the wind, temperature and humidity parameters corresponding to the two locations and the available observation times. The model results were compared with corresponding observations for validation. Observations clearly show the variations of the mixed layer with gradual increase of daytime. Due to the high resolution of the observations, the decrease of temperature with height up to the top of the mixed layer, capped by sudden increase of temperature and dryness in a thin vertical layer could be clearly delineated. The model simulated an inversion at the top of boundary layer (around 900 m height) and its gradual weakening with surface heating of daytime period. The model simulated wind variations with height at the two locations were also validated with observations. The heights of the PBL, LCL and CAPE were computed using the observations and the model predicted variables at the two different locations and at different times and were compared. All these features are noted to have good correspondence with the observations.